This study was a multicenter, blinded, randomized withdrawal study in subjects with primary Restless Legs Syndrome (RLS). Eligible subjects were initially enrolled in a 24-week single blind (SB) treatment period during which they received XP13512. Subjects who completed the initial treatment period and met the responder criteria were then randomized 1:1 to receive either XP13512 or placebo during the 12-week double-blind (DB) treatment period. The primary study objective was to assess the maintenance of efficacy of XP13512 1200 mg taken once daily in the long-term treatment of subjects with primary RLS. The secondary study objectives were to assess maintenance of improvements in sleep outcomes and quality of life, and to assess the safety and tolerability of XP13512 in the treatment of primary RLS subjects.